**Core Concept**
Monoamniotic monochorionic twins are a type of twinning that occurs when a single fertilized egg splits to form two embryos, but this split happens after the formation of the **amnion**, a membrane that encloses the embryo. This leads to the development of two fetuses that share both a **placenta** and an **amniotic sac**.

**Why the Correct Answer is Right**
The correct answer is related to the timing of the split in the development of monozygotic (identical) twins. If the split occurs after the formation of the **inner cell mass** but before the formation of the **amnion**, around 8 days post-fertilization, it results in monochorionic-diamniotic twins. However, if the split occurs even later, specifically after the 8th day, the twins will share the same amniotic sac, resulting in **monoamniotic monochorionic twins**. This usually happens if the division occurs after the **amnion** has formed.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because division at this stage would result in dichorionic-diamniotic twins.
**Option B:** Also incorrect as it doesn't specify the correct timing for monoamniotic monochorionic twins.
**Option C:** Incorrect as the specific timing for monoamniotic monochorionic twins is after the amnion has formed.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that monoamniotic monochorionic twins have a higher risk of complications, including **entanglement of umbilical cords** and **twin-to-twin transfusion syndrome**, due to sharing the same amniotic sac and placenta.

**Correct Answer:** D. after 8 days